Bright silver electroplating

ABSTRACT

FULLY BRIGHT SILVER ELECTRODEPOSITS ARE OBTAINED FROM OTHERWISE CONVENTIONAL CYANIDE-TYPE SILVER PLATING ELECTROLYTES IN THE PRESENCE OF SMALL AMOUNTS OF ACIDS OF THE FORMULA   R-CX-NR&#39;&#39;-NH-CS-SH   AND OF THEIR SOLUBLE SALTS, IN THE FORMULA R BEING LOWER ALKOXY OR NHR&#34;; R&#34; BEING HYDROGEN, LOWER ALKYL, PHENYL, HALOPHENYL, LOWER ALKYL PHENYL, OR LOWER ALKOXY PHENYL; R&#39;&#39; BEING HYDROGEN OR LOWER ALKYL; AND X BEING OXYGEN, SULFUR, OR IMINO, BUT R BEING DIFFERENT FROM AMINO WHEN X IS SULFUR.

United States Patent 3,580,821 BRIGHT SILVER ELECTROPLATING Hans-Gunther Todt and Wolfgang Dahms, Berlin, Germany, assignors to Schering A.G., Berlin, Germany No Drawing. Filed June 24, 1969, Ser. No. 838,040 Claims priority, application Germany, June 27, 1968,

P 17 71 699.3 Int. Cl. C23b /26, 5/46 US. Cl. 204-46 5 Claims ABSTRACT OF THE DISCLOSURE Fully bright silver electrodeposits are obtained from otherwise conventional cyanide-type silver plating electrolytes in the presence of small amounts of acids of the formula and of their soluble salts, in the formula R being lower alkoxy or NHR"; R" being hydrogen, lower alkyl, phenyl, halophenyl, lower alkyl phenyl, or lower alkoxy phenyl; R' being hydrogen or lower alkyl; and X being oxygen, sulfur, or imino, but R being different from amino when X is sulfur.

This invention relates to the electrodeposition of silver from aqueous electrolytes in which the silver is present mainly as a complex cyanide, and more particularly to electrolytes for bright silver plating.

Many brighteners for silver plating baths of the complex cyanide type are known, and some are in common use. The known brighteners have at least one of the following shortcomings:

(1) The silver deposit formed is not mirror bright, and needs at least some mechanical polishing for adequate appearance.

(2) The cathode current density range in which fully bright deposits can be obtained is rather narrow so that a coating of uniform appearance cannot be produced on deeply recessed objects.

(3) The brightener is a complex organic compound which cannot be purified at practical cost. The commercial product therefore contains by-products which are at best inert and at worst reduce the brightening effect. The amount and nature of the by-products cannot be held constant from one batch to the next so that difficulties in dosing the brightener are experienced by the plater.

The primary object of the invention is the provision of uniformly bright silver electrodeposits directly from a cyanide type plating bath on complexly shaped objects in a convenient and simple manner.

It has been found that brighteners of the formula and their water-soluble salts, when added to otherwise conventional electroplating baths of the cyanide type in small amounts, produce bright electrodeposits directly from the bath over an unusually wide range of cathode current densities. The compounds are readily prepared in pure form so that difierent batches of the brighteners can be made of uniform composition without undue expense.

In the formula, R may be lower alkoxy or NHR", and R" is hydrogen, lower alkyl, phenyl, halophenyl, lower alkyl phenyl, or lower alkoxy phenyl. R is hydrogen or lower alkyl, and X is oxygen, sulfur, or the imino group (NH), but R is difierent from amino when X is sulfur.

The compounds of the invention are equally effective when added to a plating bath in the form of the free acid or in the form of a soluble salt such as the silver salt, an alkali metal salt, or more specifically a salt of sodium or potassium, or as a salt of ammonium or its alkyl deriva- 3,580,821 Patented May 25, 1971 "ice tives. The brightening effect is due solely to the anionic component.

The following compounds are illustrative of the brighteners in the silver plating baths of the invention and are identified hereinbelow by capital letters for greater convemence:

(A) Potassium N-carbamy1-hydrazine-N'-dithiocarboxylate, H N-CONHNH-CSSK (B) Triethylammonium N-methylthiocarbamyl-hydrazine-N'-dithiocarboxylate,

(C) Potassium N methylthiocarbomyl hydrazine-N'- dithiocarboxylate, CH NHCS-NHNHCSSK (D) Triethylammonium N methylthiocarbamyl N- methyl-hydrazine-N-dithiocarboxylate,

CHsNHC S-NNH-C s SNH CH Ha (E) Triethylammonium N-phenylthiocarbamyl-hydrazine-N'-dithiocarboxylate,

C H -NHCS-NH-NHCSSNH (C H 3 (F) Potassium N- carbomethoxy-hydrazine-N'-dithiocarboxylate, CH -OCO-NH-NH-CSSK (G) Sodium N-guanyl-hydrazine-N-dithiocarboxylate, H NCNH-NHNHCSSNa (H) Sodium N methylguanyl-hydrazine-N'-dithiocarboxylate, CH NH-CNH--NHNHCSSNa 30 (I) Sodium N-ethylguanyl-hydrazine-N-dithiocarboxylate, C H -NHCNHNHNHCSSNa (J) Potassium N-p-bromophenylthiocarbamyl hydrazine-N'-dithiocarboxylate,

BrC H -NH-CSNH-NHCSSK (K) Potassium N-p-ethoxyphenylthiocarbamyl hydrazine-N'-dithiocarboxylate,

oxan. The following example is merely illustrative of the basically known procedure.

Example 1 50 ml. of a l-molar solution of potassium hydroxide in ethanol (0.05 mole KOH) were heated with 0.05 mole methylthiosemicarbazide. At 40 C., a solution of 0.07 mole carbon disulfide in 0.5 mole ethanol was added, and the resulting mixture was vigorously refluxed for ten minutes. Upon cooling to 40 C., a crystalline precipitate of Compound C was formed. It was recovered by suction filtration and purified by dissolving the crystals in a little water and adding ethanol to induce crystallization.

The other derivatives of hydrazinedithiocarboxylic acid listed above were prepared in an analogous manner.

The compounds of the invention are added to otherwise conventional cyanide silver plating solutions in amounts that may vary between 3 milligrams and 3 grams per liter, preferably between 10 milligrams and 1 gram per liter, depending on the brightener employed and operating conditions. They dissolve readily. Optimum amounts, which usually are not critical, are readily determined in Hull cell tests.

The plating solutions employed typically contain 20 to 50 g./l. silver in the form of a complex cyanide, 50- 200 g./l. free cyanide as the potassium salt or an equivalent amount of the sodium salt, and -100 g./l. K 00 They may further contain known organic and inorganic secondary brightening agents such as thiocarbonates (sodium trithiocarbonate or ethylene trithiocarbonate), xanthogenates (potassium xanthogenate), thiocarbamates (ammonium thiocarbamate), organic selenium compounds (piazselenole), also inorganic selenium compounds (potassium selenocyanate or sodium selenite), and compounds of metals of Group Va of the Periodic System such as antimony compounds. Surfactants and colloids conventionally employed in cyanide type silver plating baths are fully compatible with the brighteners of the invention.

The brighteners of the invention are most eifective at temperatures between about and 35 0., best results being achieved most readily at about C. Good brightness is found in cathode areas in which the actual current density is between 0.05 to 3 amps./dm. full brightness being usually available from 0.1 to 2. amps/dmfl.

Typical plating baths of the invention and their use are illustrated by the following examples.

EXAMPLE 2 A silver plating solution of the composition:

35 g./l. silver as potassium silver dicyanide 160 g./l. free potassium cyanide 30 mg./l. selenium as potassium selenocyanate produced bright silver deposits in a Hull cell test only at cathodecurrent densities of 1.6 to about 4 amps./dm. No significant improvement could be achieved by the addition of the usual surfactants (wetting agents). When potassium N-carbamyl-hydrazine N dithiocarboxylate (Compound A) was added in an amount of 200 mg./l., the lower limit of the bright plating range was shifted to 0.1 amps.

EXAMPLE 3 A conventional silver plating solution containing 30 g./l. sodium silver dicyanide and 100 g./l. free sodium cyanide produced dull electrodeposits over the entire operating range of a standard Hull cell. When 20 mg./l. Compound B were dissolved in the bath, a fully bright deposit of silver was obtained from approximately 0.1 to approximately 4 amps./dm.

EXAMPLE 4 A silver plating electrolyte was prepared to the following composition:

35 g./l. silver as potassium silver dicyanide 160 g./l. free potassium hydroxide 30 mg./l. selenium as potassium selenocyanate A sample of the bath gave bright deposits in at Hull cell test between about 1.6 and 4 amps./dm. When 100 mg./l. Compound G were added, the lower limit of the bright range dropped to less than 0.1 amp/dmfi.

EXAMPLE 5 A conventional bright silver plating bath (French Pat. 1,475,290) was prepared to the following composition:

30 g./l. silver as potassium silver dicyanide g./l. free potassium cyanide 100 mg./l. potassium thiosemicarbazidedithiocarboxylate A Hull cell test showed a haze at current densities below about 1 amp./dm. This haze was clearly visible and detracted from the appearance of plated work particularly after lacquering. When 100 mg./l. of Compound C were substituted for the brightener of the French Patent, no haze was found at current densities as low as 0.4 amp./dm.

EXAMPLE 6 A silver plating electrolyte containing 35 g./l. silver in the form of potassium silver dicyanide g./l. free potassium cyanide 30 g./l. potassium carbonate yielded mat deposits at all current densities. Fully bright deposits over the entire range of usual cathode current densities (0.1-4 amps./dm. and partly to less than 0.1 amp./dm. were achieved by the addition of any one of the brighteners E,J,K,L,M,N,O, or P in an amount of 150 mg./l. and in the presence of 1 g./l. of polypotassium sulfonaphthyl methylene or other wetting agents.

The electrolytes of the invention have been found to be particularly useful in the electrodeposition of thin silver coatings on hollowware over a layer of bright nickel. No mechanical polishing was found to be necessary. In a typical application, cups, dishes and similar articles made from copper or copper alloys were carefully cleaned and electrolytically degreased. They were then electroplated with bright nickel, rinsed well, pickled, flash silvered for 5 seconds at about 5 amps./dm. and transferred without rinsing to a silver plating bath of'the invention in which they were coated at an average current density of 1 amp./dm. with cathode agitation. Mirror bright silver deposits free from haze were obtained at coating thickness of as small as 1 micrometer.

What is claimed is:

1. An aqueous alkaline silver plating bath containing silver in the form of a complex cyanide, free cyanide, and an amount of an acid of the formula or of a dissolved salt of said acid, in said formula R being lower alkoxy or NHR"; R" being hydrogen, lower alkyl, phenyl, halophenyl, lower alkyl phenyl, or lower alkoxy phenyl; R being hydrogen or lower alkyl; and X being oxygen, sulfur, or imino; R being different from NH when X is sulfur; said amount being between 3 milligrams and 3 grams per liter of said bath.

2. An aqueous alkaline silver plating bath as defined in claim 1 wherein said acid amount is between 10 milligrams and 1 gram per liter.

3. A method of electrodepositing bright silver on a conductive object which comprises making said object the cathode in the bath defined in claim 1.

4. A method of preparing a bright silver plating solution which comprises adding 3 milligrams to 3 grams per liter of an acid of the formula or of a water soluble salt of said acid to an aqueous plating bath containing, per liter, 20 to 50 grams silver as a complex cyanide, 50-200 grams free alkali metal cyanide, and 0 to 100 grams alkali metal carbonate, in said formula R being lower alkoxy'or NHR"; R" being hydrogen, lower alkyl, phenyl, halophenyl, lower alkyl phenyl, or lower alkoxy phenyl; R being hydrogen or lower alkyl; and X being oxygen, sulfur, or imino; R being different from NH when X is sulfur.

References Cited UNITED STATES PATENTS 5/1969 Korpiun et a1. 204-46 6 FOREIGN PATENTS 959,775 3/1957 Germany 204-46 1,125,165 8/1968 Great Britain 204-46 5 GERALD L. KAPLAN, Primary Examiner US. Cl. X.R.

204DIG 2; 260513.5, 553 

